TiO2-catalyzed photooxidation of arsenite to arsenate in aqueous samples

The TiO2-catalyzed photooxidation of arsenite (As(III)) to arsenate (As(V)) was studied in aqueous TiO2 suspensions using a solar simulator which emitted ultraviolet and visible radiations. The concentration of As(III) was varied between 50 microg l(-1) and 10 mg l(-1), and the concentration of TiO2 between 1 mg l(-1) and 50 mg l(-1). Total oxidation of As(III) to As(V) occurred within minutes. The concentration of As(III) declined exponentially which indicates first-order kinetics. In the pH range between 5 and 9 there was no significant influence of the pH of the suspension on the reaction rate. Batch experiments without irradiation showed that part of the arsenic was adsorbed on the TiO2 surface. When using 100 microg l(-1) As and between 1 mg l(-1) and 50 mg l(-1) TiO2, 8-39% of As(III) and up to 73% of As(V) were adsorbed by TiO2. As(III) was also oxidized by UV radiation in the absence of TiO2, but the reaction was slower than in the presence of TiO2 resulting in an irradiation time too long for practical use. In addition, oxidation of As(III) in the presence of TiO2 was also observed under solar irradiation within a few minutes.